Once an ink has been deposited on a print medium it takes a certain time for the printed image to dry. Drying is driven in part by evaporation and in part by absorption into the print medium. Short xe2x80x9cdrytimesxe2x80x9d (e.g., less than about 1 s) are generally preferred in the printing industry as they reduce surface bleed, reduce the potential for smudging, and shorten the overall print time. The drytime of an image is influenced by a number of factors that include the chemical composition of the deposited ink and the physical and chemical characteristics of the print medium.
The rate at which an ink is absorbed into a print medium may, for example, be affected by the surface tension and viscosity of the ink. Inks with low surface tension and viscosity are absorbed more quickly into print media. Alcohols such as diols and glycols have low viscosities and low surface tensions and are therefore frequently added to ink compositions to enhance ink absorption. Surfactants are also capable of reducing the surface tension of inks and are further capable of providing a stabilizing interface between hydrophilic and hydrophobic environments. However, reducing the surface tension and viscosity of an ink increases the risk of surface bleed and may therefore have an adverse effect on print quality.
Uncoated print media are porous and therefore absorb ink fairly rapidly through capillary action. In contrast, coated print media have smooth non-porous surfaces that are less permeable to inks. As a consequence of this physical difference, when inks are deposited on coated print media they generally take longer to dry than when they are deposited on uncoated print media. The situation is further worsened when the ink and print medium are chemically incompatible (e.g., when a hydrophilic ink is deposited on a hydrophobic coating).
The coatings used in the printing industry are generally pigment rich formulations that provide improved gloss, slickness, color, printing detail, and brilliance. The pigment (e.g., china clay, barium sulfate, calcium carbonate, titanium oxide, etc.) is similar to a filler that fills the pores on the surface of the print medium. The pigments are mixed with a variety of additives (e.g., dispersants, thickeners, preservatives, defoamers, dyes, etc.) and adhesives that hold the coating to the surface of the print medium and provide a suitable finish and rub-resistance. Commonly used adhesives are latex based and include natural adhesives (e.g., starches and proteins such as casein and soya extract) and synthetic adhesives (e.g., vinyl acetate, vinyl alcohol, acrylic, and styrene-butadiene based polymers).
Water based inks are generally preferred in the ink-jet printing industry because water is readily available at low cost, chemically unreactive, non-toxic, environmentally friendly, and because the physical properties of water (e.g., boiling point, viscosity, surface tension, etc.) are ideally suited to the thermal and piezoelectric ejection of fine droplets of ink. Adhesives used in coatings for ink-jet print media are therefore usually made from water compatible hydrophilic polymers (e.g., polyvinyl alcohol).
In offset printing, an image is transferred from a printing plate (positive image) to a rubber blanket (negative image) and then to a substrate (as a positive once again). The ink-receptive image areas on the printing plate are made from a hydrophobic material, while the ink-repellent non-printing areas are made from a hydrophilic material. Inks that include hydrophobic solvents such as xylene, toluene, linseed oil, soybean oil, etc. are therefore preferred in the offset printing industry, and adhesives used in coatings for offset print media are typically made from hydrophobic polymers (e.g., styrene-butadiene based polymers).
It will be appreciated from the above discussion that commercially available ink-jet inks and commercially available offset coated print media are chemically incompatible. Furthermore, the physical properties (e.g., high viscosity) and the chemical reactivity of commercially available offset inks are not suitable for ink-jet printing methods and not compatible with ink-jet printing equipment. It would be desirable to provide ink compositions and ink-jet printing methods that allow inks to be printed by ink-jet methods on commercially available offset coated papers and hydrophobic print media in general. In particular it would be desirable to provide ink compositions that perform well in ink-jet printers on both a short and long term and show reasonable drytimes when printed on hydrophobic media.
The present invention addresses and solves the above-mentioned problems by providing ink compositions that include a water soluble dye and a vehicle, the vehicle including water, a glycol ether, a humectant, and a non-ionic surfactant. The present invention further provides fixer compositions for stabilizing inventive inks on hydrophobic media that include a fixing agent and a vehicle, the vehicle including water, a glycol ether, a glycol ether ester or mixture thereof, a humectant, and a non-ionic surfactant.
The present invention further addresses and solves the above-mentioned problems by providing methods of printing on hydrophobic media that include the steps of providing an ink that includes a water soluble dye and a vehicle, the vehicle including water, a glycol ether, a glycol ether ester or mixture thereof, a humectant, and a non-ionic surfactant; providing a hydrophobic print medium; and depositing the ink on the hydrophobic print medium. The method of printing may further include steps of applying heat to the hydrophobic print medium before, during and/or after the ink deposition step. The method of printing may yet further include steps of providing a fixer that includes a fixing agent and a vehicle, the vehicle including water, a glycol ether, a glycol ether ester or mixture thereof, a humectant, and a non-ionic surfactant; and depositing the fixer on the hydrophobic print medium before, after, or both before and after the ink is deposited.